[PATCH] D77807: [LLD][ELF] Implement --discard-* for cases when -r or --emit-relocs are used.

[Igor Kudrin via Phabricator via llvm-commits]

ikudrin added a comment.

In D77807#1974679 <https://reviews.llvm.org/D77807#1974679>, @MaskRay wrote:

> Honestly I am wondering whether a relocation scanning is justified. The only problem I can think of with the current conservative choice for --discard-locals + (-r | --emit-relocs) is that the output is larger. The output contains some otherwise unneeded STB_LOCAL symbols. Note, STB_LOCAL symbols not used by a relocation don't interfere with linking.


In our case, the output is analyzed with additional tools and these unneeded symbols slow down the process significantly.

> This discards local symbols as well. My question is why we have these unreferenced local symbols in object files and why we need to save .symtab space for them.
> 
> An assembler can convert a local symbol reference to a relocation referencing a STT_SECTION symbol, or resolve the fixup at assembly time. If the intention is to discard the unreferenced local symbols, a better way may be to not generate the local symbols in the object files. Alternatively, generate `.L` temporary labels which are a special kind of local symbols which usually don't survive in the object file.

Here is a reduced example of the initial problem I am solving in this patch:

  $ cat a.c
  static int foo = 0;
  int bar() { return foo; }
  $ clang -c a.c
  $ readelf -rs a.o
  Relocation section '.rela.text' at offset 0x150 contains 1 entry:
    Offset          Info           Type           Sym. Value    Sym. Name + Addend
  000000000007  00040000000b R_X86_64_32S      0000000000000000 .bss + 0
  ...
  Symbol table '.symtab' contains 6 entries:
     Num:    Value          Size Type    Bind   Vis      Ndx Name
  ...
       2: 0000000000000000     4 OBJECT  LOCAL  DEFAULT    4 foo
  ...
  $ ld.bfd -r a.o --discard-all -o ar.o && nm ar.o
  0000000000000000 T bar
  $ ld.lld -r a.o --discard-all -o ar.o && nm ar.o
  0000000000000000 T bar
  0000000000000000 b foo


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